Terahertz (THz) technologies utilize electromagnetic radiation generally in the frequency range between 100 GHz and 10 THz (i.e., wavelengths of 3 mm to 30 μm, energies of 0.4 to 40 meV, or equivalent blackbody radiation temperatures of 5 K to 500 K). Terahertz technologies have many potential applications in diverse fields, including space and atmospheric sciences, molecular spectroscopy, remote sensing, biology, medical imaging, and communications.
Historically, there has been much interest in terahertz technologies for high-resolution spectroscopy for space, planetary, and Earth science. For example, much of the interstellar medium radiates in the terahertz region, somewhat above the cosmic microwave background, enabling terahertz measurements to probe star formation and the early universe. Planetary atmospheres have background temperatures of tens to several hundred degrees Kelvin, enabling terahertz observation of extraterrestrial atmospheric conditions. Furthermore, thermal emission lines in the terahertz region from gases in the Earth's stratosphere and upper troposphere provide important indicators of ozone destruction, global warming, and pollution.
Metals and polar liquids, such as water, are opaque in the terahertz region. However, many non-metallic materials that are visually opaque are partially transparent or exhibit molecular resonances in the terahertz region. For example, many solids, notably oxides and organic solids, have distinct series of optically-active lattice vibration modes that cause large optical absorption resonances in the range 1 to 10 THZ. While such lattice vibration resonances are relatively broad, if the “signatures” are known they can be used to identify the solid.
In the gas phase, molecular spectral signatures can be used to detect and identify molecules with high speed, sensitivity, and very high specificity. Many molecules, from simple diatomic chemicals to complex macromolecules, have stronger and more distinctive absorption and emission resonances in the terahertz region than in either microwave or near infrared-to-visible ranges. Especially at lower pressures, smaller molecules generally have very sharp terahertz signatures, with Doppler limited widths around 1 MHz, providing significantly enhanced spectral resolution as compared to infrared signatures. Nearly all non-centrosymmetric molecules have resonances between 0.1 to 10 THz that are predominately rotational modes or hybrid rotational-vibrational modes, determined by a molecule's moment-of-inertia. Since the moment-of-inertia depends on the distribution of mass within the molecule as well as total mass, a spectrum based on moment-of-inertia can discriminate between molecular species better than spectroscopy based only on mass. Compared to microwave spectroscopy, interaction strengths are generally larger in the terahertz region since the strength of the interaction increases by greater than the square of the frequency, until reaching a peak in the terahertz region that is specified by the molecular mass. Such high molecular absorption/emission cross-sections can provide potentially excellent signal-to-noise detection levels and thus high sensitivity to trace concentrations of molecules. Further, at high spectral resolution each molecular signature is unique enough that only a few lines are generally needed to identify a molecule. Therefore, rotational absorption spectroscopy can be used to unambiguously identify molecules in the gas phase, even in the presence of contaminants, whereas vibrational spectroscopy lacks such molecular specificity. Further, for gas molecules that are at a higher temperature than the background, the characteristic spectral emission features will be observable passively as well as actively. Therefore, the terahertz region has enormous potential in the area of remote spectroscopy with unprecedented, unsurpassed species discrimination capability and minimized probability of error due to either missed detection or misidentification.
Biological identification using optical techniques typically uses either standoff detection, using some specific spectroscopic signature, for example, native fluorescence upon UV excitation or infrared-absorption/Raman signatures, or contact techniques, wherein the target biomolecule is made to react with other chemical compounds or biomolecules, and further fluorescent tagging makes the detection possible. An example of this later method is the sandwich immunoassay for protein detection. For both methods, the terahertz region of the electromagnetic spectrum is attractive for label-free detection of biomolecules, since there is a very high density of vibrational-rotational modes that can be accessed through direct absorption or other dielectric coefficient sensing techniques.
For example, the vibrational spectrum at low frequencies in the range of 0.1-3 THz is particularly sensitive to structural changes of proteins, such as conformational changes that occur upon protein capture by antibodies and other hybridization reactions. As a result, guided-wave terahertz biosensors can be used effectively for label-free biosensing. Examples of established optical techniques for label-free biosensing are surface plasmon resonance (SPR), surface acoustic wave (SAW) detection. These techniques usually rely on measuring the change of a material property caused by a protein-ligand interaction with great sensitivity. In SPR, the adsorbed layers cause a change in refractive index whereas in SAW (or quartz crystal monitoring) detection it is a pure mass loading change. Conversely, a terahertz biosensor can be sensitive to the conformational changes that occur upon protein-ligand binding.
A highly desirable application of terahertz technology is to do imaging of objects at useful standoff distances in real time for object or pattern recognition. Since terahertz waves have a much smaller wavelength than microwaves, they can provide sub-millimeter spatial resolution. Because terahertz irradiation does not involve the health and safety issues of ionizing radiation, such as are a concern with X-ray imaging, applications of terahertz technologies may include noninvasive tomographic imaging or spectroscopic characterization of biological materials. A primary advantage of terahertz imaging for security applications is that many materials that are completely opaque or highly reflective at microwave and/or infrared-visible frequencies are at least partially transparent in the terahertz. Therefore, because terahertz radiation is nondestructive and can penetrate non-metallic and non-polarizing external coverings (e.g., clothing, paper, wood, and plastics), the technology may be useful in security screening for hidden explosives and concealed weapons. Terahertz technologies can be used to either detect or preferably image something otherwise hidden behind such a covering material, using either active or passive methods. Finally, terahertz imaging may also be useful for industrial processes, such as package inspection and quality control.
Finally, terahertz signals can have a high bandwidth and are potentially useful for free-space communications. Therefore, terahertz technologies may be useful for space-based communications, such as satellite-to-satellite. However, limited atmospheric propagation, due to water and oxygen absorption, has discouraged the development of terahertz technologies for radar and terrestrial communications. Nonetheless, the technology may be attractive for relatively secure short-range communications, such as wireless communications in situations in which short-burst beamed messages with limited broadcast range is desirable. A terahertz carrier has a higher frequency than microwaves, enabling higher data rates (e.g., about 10 GB/sec). Further, the shorter wavelength enables higher directionality.
However, beyond basic science, these applications in the terahertz region are relatively undeveloped. Much progress is still required to provide field-deployable terahertz systems, especially for military, anti-terror, and biomedical imaging applications. Terahertz applications remain relatively undeveloped because the terahertz region lies between the traditional microwave and optical regions of the electromagnetic spectrum, where electronic or photonic technologies have been developed for these purposes. Terahertz applications have been hampered due to the historically poor performance of terahertz components lying in the “technological gap” between these traditional electronic and photonic domains. Furthermore, atmospheric opacity is strong in the terahertz, so that terahertz components must meet extremely stringent signal-to-noise requirements for any in-atmosphere stand-off application, unfortunately also in a frequency range where conventional technologies have inadequate performance.
In particular, the generation and detection of electromagnetic fields at terahertz frequencies has been difficult. To date, active terahertz generators have only demonstrated relatively low power capability. Traditional electronic solid state sources based on semiconductors are limited by electron transit times and roll-off at severely high frequencies (i.e., limited to less than about 0.1 THz). Tube sources are difficult to scale to small sizes, due to the extremely high fields and current densities required. Traditional semiconductor diode lasers are limited by normal optical bandgaps to frequencies greater than about 25 THz. Therefore, frequency conversion techniques have typically been used to reach terahertz frequencies, including upconversion of millimeter waves using electronic or multiple harmonic techniques, or downconversion from the visible or near-infrared using frequency mixing/switching or nonlinear optical processes. However, continuous microelectronic sources using either microwave upconverters or infrared downconverters have difficulty exceeding 1 μW average power at around 2 to 3 THz.
Semiconductor lasers are inherently compact and durable, due to their internal, electrically-pumped optical cavity and low power requirement. Recently, terahertz sources based on quantum cascade lasers (QCLS) have produced relatively high power in a compact size. QCLs are the first semiconductor sources of terahertz radiation capable of average powers in a pulsed mode in excess of 250 mW at cryogenic temperatures. QCLs are unipolar semiconductor devices comprising complex layered heterostructures of two or more semiconductor alloys forming an active waveguide core, typically mounted on a metallic heat-sink. The complex QCL structure can be grown by molecular beam epitaxy (MBE). MBE enables accurate control of the sub-nanometer semiconductor layers with high reproducibility over hundreds of periodic layers.
Quantum cascade lasers rely on the emission from transitions between subbands in a quantum well. Light is produced in an active region by intersubband transitions of a single charge carrier (i.e., an electron) between two quantized levels in the conduction band. In a QCL biased at an operating voltage, a photon is emitted by an intrawell transition between an upper level and a lower level in an active region. To achieve population inversion for lasing, electrons must be injected rapidly into the upper level and then rapidly extracted from the lower level and tunnel into the upper level of the down-stream active region. To maximize the gain, tens to hundreds of these active regions can be cascaded together, enabling electrons that are recycled from one active region to the next to emit more than one photon per pass through the device, enabling high emission power. Because the energy difference between the two quantized levels is determined by the specific structure design (i.e., the quantum well and barrier widths), the laser can be band-structure engineered to emit at any wavelength within a broad spectral range. To minimize device loses and confine the terahertz radiation to the gain material, the active region can be inserted into a waveguide. Laser action requires that the gain be adequate to overcome device losses, primarily due to free-carrier losses in the waveguide and mirror losses. Many variations on this basic scheme have evolved.
However, the weak radiation output from passive and traditional active terahertz sources, the low photon energies of terahertz radiation, and high atmospheric attenuation due to molecular absorption (e.g., water vapor) frequently results in a weak received terahertz signal that may be difficult to distinguish from noise. Therefore, terahertz detection can also be difficult. Current terahertz detectors include both direct and heterodyne detectors.
Direct detectors generally directly convert the received power to a voltage or current that is proportional to the incoming power. Examples of direct detectors include rectifiers, bolometers, and pyroelectrics. A common direct detector uses antenna coupling to a small area Schottky diode that responds directly to the terahertz electric field. Detection depends on the nonlinear rectification properties of a metal-semiconductor junction. Advantages of the Schottky diode include a useful sensitivity over a large wavelength range, large instantaneous bandwidth, excellent performance at room temperature, and ease of fabrication.
For shorter wavelengths (i.e., frequencies above 1 THz), direct detectors generally have good responsivity and are sensitive to a broad band of frequencies. However, direct detectors generally provide no frequency discrimination, unless they are coupled with an external resonator or interferometer. Furthermore, they are sensitive to incoherent background noise and interference. Finally, direct directors are typically very slow, with 1 to 10 ms response times required to obtain an adequate signal-to-noise. Therefore, direct detectors have been used mainly for wideband applications, such as thermal imaging.
Heterodyned detection is desirable for some terahertz applications. Especially at low pressures such as a space environment, terahertz signatures of many molecules are very unique, enabling identification even with only a few spectral lines over a narrow spectral region. However, because the emission lines can be quite narrow and may be closely spaced, high resolution spectroscopy is desirable to take full advantage of terahertz discrimination capabilities. High-resolution heterodyne detection covering the frequency intervals of expected signatures is highly desirable for these applications. Further, particularly for weak signals, heterodyning can be used to coherently downconvert the terahertz signal to increase signal-to-noise by reducing bandwidth. The downconverted signal can then be post-amplified and processed using conventional microwave techniques.
Heterodyne mixers beat the signal RF frequency against a known local oscillator (LO) frequency to generate an intermediate frequency (IF) difference signal that is tunable through the LO frequency. The LO can have a fixed output power that is generally much greater than the power of the received RF signal. A nonlinear mixer produces an IF output power that is proportional to the product of the powers of the received RF signal and the LO signal. Mixers display good rejection of incoherent noise and interference. They are typically fast, with IF bandwidths of 0.1 to 10 GHz. Furthermore, narrowband detectors do not require additional frequency selective elements to analyze the spectrum of the incoming terahertz radiation as long as the received RF signal is within an IF bandwidth of the LO frequency. Therefore, heterodyne detectors have been used in narrow frequency band, high-resolution applications at lower terahertz frequencies, such as for molecular spectroscopy. Common mixers are field-type devices that have a strong quadratic nonlinearity.
However, a fast solid-state terahertz radiation mixer is still needed to enable coherent detection for terahertz applications requiring high resolution. In particular, a microelectronic-based integrated heterodyne terahertz transceiver is highly desirable for field-deployable applications. Such a transceiver requires the successful integration of both terahertz detectors and sources on a single chip, along with cooling, optics and control electronics, while maintaining high source power (e.g., greater than 10 mW), detection sensitivity, and operating temperatures, all in a compact, reliable, integrated package.